A reference post rather than a discussion. Corrections are the point; I would rather this be right than mine. It is about compounded supply, and it is deliberately narrow — everything I am not confident about is marked as such.
What is actually established
They are two different exemptions from the same federal requirements and they buy different things. A 503A pharmacy is regulated primarily by the state board, needs a patient-specific prescription, is exempt from CGMP, and may use a bulk substance that has a USP monograph, is a component of an approved drug, or appears on the 503A bulks list — three independent doorways. A 503B outsourcing facility registers with the FDA, is inspected on a risk basis, must comply with CGMP, may compound for office stock without a patient-specific prescription, and has one doorway to a permitted bulk substance: the 503B bulks list, or the drug shortage list.
The condition it depends on
The enforcement dates were staggered by category — 503A first, 503B a few weeks later — because outsourcing facilities have manufactured inventory and clinic contracts to unwind while a 503A makes to order.
The practical version
Two things anyone can check: a state licence number for a 503A, and an FDA outsourcing-facility registration for a 503B. Both are publicly searchable, and a pharmacy unwilling to give you either has answered the question.
What I am not sure about
The narrow version of the question is what actually distinguishes 503A from 503B, in terms of what each may make and from what starting material. Not looking for reassurance. Looking for the part I have got wrong.
— KevinCompounds · corrections welcome and will be edited into this post with credit